JP2014238960A - Metal oxide, negative electrode active material for sodium ion battery, negative electrode for sodium ion battery, and sodium ion battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode active material which allows a sodium ion battery having high safety and good battery performance to be manufactured.SOLUTION: A metal oxide comprises a lamellar crystal structure. The crystal structure has a transition metal layer including quadrivalent titanium. Using the metal oxide, a sodium ion battery having high safety and a good battery performance can be manufactured.

Description

  The present invention relates to a metal oxide having a layered crystal structure, a negative electrode active material for a sodium ion battery, a negative electrode for a sodium ion battery, and a sodium ion battery using the metal oxide.

Lithium secondary batteries are high energy density secondary batteries that have already been put into practical use as small power sources for mobile phones, notebook computers and the like. Lithium secondary batteries have already been put into practical use as compact power sources for electric vehicles, hybrid vehicles, and the like. In addition, since lithium secondary batteries can be used as large-scale power sources such as power sources for automobiles such as electric vehicles and hybrid vehicles, and distributed power storage power sources, the demand is increasing.
However, since a rare metal element such as lithium is used in the lithium secondary battery, there is a concern about unstable supply of the rare metal element when the demand for the lithium secondary battery increases.

Therefore, recently, attention has been focused on sodium batteries using sodium, which is more abundant than lithium, and research on materials such as a positive electrode active material and a negative electrode active material used in sodium batteries is also underway.
For example, Patent Document 1 describes an example in which Na ₂ Ti ₃ O ₇ having a zigzag layered structure is used as a negative electrode material for a sodium ion battery.
Patent Document 2 describes that Y ₂ Ti ₂ O ₅ S ₂ having a Ruddleden-Popper structure is used as an active material for a sodium ion battery, and describes that precipitation of metal Na can be suppressed. ing.
Further, Non-Patent Document 1, it is described that utilize _Na x TiO ₂ type O3 type structure (0.7 <x <1) as an active material for a sodium ion battery.

International Publication No. 2013/004957 JP 2013-062121 A

J. et al. Inclusion Phenomena, 1, 45-51 (1983)

As a typical negative electrode active material of a sodium ion battery, a carbon material such as hard carbon is known. However, in the case of a carbon material, metal Na is likely to be deposited, and there is a problem in safety. On the other hand, for example, composite metal oxides such as Na ₂ Ti ₃ O ₇ described in Patent Document 1 can be used as the negative electrode active material, but these materials are irreversible caused by the first charge / discharge. At present, the capacity is large and sufficient performance as a secondary battery cannot be obtained.
That is, this invention makes it a subject to provide the negative electrode active material which can manufacture a sodium ion battery provided with high safety | security and favorable battery performance.

As a result of intensive studies to solve the above problems, the present inventors have found that a metal oxide having a layered crystal structure and further having a transition metal layer containing tetravalent Ti in the crystal structure is sodium. It has been found that a sodium ion battery having a high safety and good battery performance can be produced by using this material, which is a very suitable material for the negative electrode active material of an ion battery, and the present invention has been completed.
That is, the present invention is as follows.

(1) A metal oxide having a layered crystal structure, wherein the crystal structure has a transition metal layer containing tetravalent Ti.
(2) The metal oxide according to (1), wherein the composition formula is represented by the following formula (I).
_{Na x A a Ti b M c} O 2 ··· (I)
(In Formula (I), x <1, 0 <a ≦ 1/3, 2/3 ≦ b <1, 0 ≦ c <1/3, a + b + c = 1. A is selected from Li and Mg. And at least one element selected from V, Cr, Fe, Mn, Co, Ni, Cu, Ca, Sn, and Zn, and constitutes a transition metal layer Ti can be substituted.)
(3) The metal oxide according to (1) or (2), wherein the crystal structure is a P2-type structure, an O3-type structure, or a P3-type structure.
(4) A negative electrode active material for a sodium ion battery (5) comprising the metal oxide according to any one of (1) to (3). A negative electrode active material for a sodium ion battery according to (4). A negative electrode for a sodium ion battery, comprising:
(6) A sodium ion battery comprising the negative electrode for sodium ion battery according to (5).

  According to the present invention, it is possible to manufacture a sodium ion battery that is excellent in safety and has a very small initial irreversible capacity. Further, since Ti is tetravalent, a sodium ion battery that is relatively stable even in air, can be easily handled and stored, and is excellent in productivity can be manufactured.

It is an XRD measurement result of the compound ₁ produced in Example _{1 (Na 0.68 Li 0.23 Ti 0.77} O 2). 2 is a charge / discharge curve of the battery produced in Example 1. FIG. Is an XRD measurement result of Example 2 was prepared with compound _{2 (Na 0.68 Li 0.23 Ti 0.77} O 2). 4 is an XRD measurement result of Compound 3 (Na _2/3 Mg _1/3 Ti _2/3 O ₂ ) prepared in Example 3. 3 is a charge / discharge curve of a battery produced in Example 3. FIG. It is an XRD measurement result of the compound 4 (Na _2/3 Mg _1/3 Ti _2/3 O ₂ ) prepared in Example 4. 4 is a charge / discharge curve of a battery produced in Example 4.

  Hereinafter, embodiments of the present invention will be described. In addition, this invention is not limited to the following embodiment.

<Metal oxide>
The metal oxide of the present invention has a layered crystal structure, and the crystal structure further includes a transition metal layer containing tetravalent Ti.
It has been reported that a composite metal oxide containing Na and Ti can be used as a negative electrode active material in a sodium ion battery, but the irreversible capacity generated by the first charge / discharge is very large in these metal oxides. At present, sufficient performance as a secondary battery cannot be obtained. The present inventors have made it irreversible by using a metal oxide having a layered crystal structure, the crystal structure of which has a transition metal layer containing tetravalent Ti, as a negative electrode active material of a sodium ion battery. It has been found that the capacity can be significantly reduced. It is presumed that this is because the structure has a stable layered structure, so that the structural change accompanying charging / discharging is small, and excessive side reactions accompanying cracking of the active material are reduced. Moreover, since such a metal oxide can be operated at a higher potential than a carbon material, there is an advantage that metal Na is not easily precipitated. Further, since Ti is tetravalent, it is relatively stable even in the air, and handling and storage become easy, and as a result, the productivity of the battery can be improved. That is, the metal oxide of the present invention is a material that is very suitable for a negative electrode active material for sodium ion batteries.
In addition, “having a layered crystal structure” and “having a transition metal layer containing tetravalent Ti” means that the transition metal atom and oxygen atom (counter anion) are arranged in a plane to form a layer structure. It means that the transition metal layer contains at least tetravalent Ti atoms.

The metal oxide of the present invention has a layered crystal structure, and the specific composition is not particularly limited as long as the crystal structure has a transition metal layer containing tetravalent Ti. What is represented by (I) is preferable.
_{Na x A a Ti b M c} O 2 ··· (I)
(In Formula (I), x <1, 0 <a ≦ 1/3, 2/3 ≦ b <1, 0 ≦ c <1/3, a + b + c = 1. A is selected from Li and Mg. And at least one element selected from V, Cr, Fe, Mn, Co, Ni, Cu, Ca, Sn, and Zn, and constitutes a transition metal layer Ti can be substituted.)
If it is a metal oxide represented by general formula (I), since rare metals, such as Y and Nd, are not used, the cost of a battery can be held down.
The specific numerical values of x, a, b and c in the formula (I) are not particularly limited as long as the above conditions are satisfied, but x is preferably 0.8 or less (x ≦ 0.8), more preferably Is 0.7 or less (x ≦ 0.7), and is usually greater than 0 (0 <x). a is preferably 0.05 or more (a ≧ 0.05), more preferably 0.1 or more (a ≧ 0.1). b is preferably 0.95 or less (b ≦ 0.95), more preferably 0.9 or less (b ≦ 0.9). Although c is not necessarily contained, it is preferably 0.2 or less (c ≦ 0.2), more preferably 0.1 or less (c ≦ 0.2) in order to keep the Ti tetravalent and stabilize the layered structure. 0.1). If it is in the said range, the sodium ion battery which has favorable battery performance can be manufactured.
M is preferably at least one element selected from V, Cr, Mn, and Fe, and more preferably at least one element selected from V, Cr, and Fe.
“A is at least one element selected from Li and Mg”, but when A is a combination of Li and Mg, b represents the total value of the ratio of Li and Mg.
Also, “M is at least one element selected from V, Cr, Fe, Mn, Co, Ni, Cu, Ca, Sn, and Zn”, but when M is a combination of a plurality of elements, , C represents the total value of the ratio of each element constituting M.
Note that M is “which can replace Ti constituting the transition metal layer”, but even if a part of the Ti layer arranged on the plane is replaced by M, the layered crystal structure is changed. It means a metal oxide that can be stably maintained. That is, for example, when a part of Ti is replaced with M, an element whose crystal structure becomes unstable and the layered crystal structure is broken cannot be M.

The metal oxide of the present invention has a layered crystal structure. Specific crystal structures include a P2 type structure, an O3 type structure, and a P3 type structure, but the specific types of crystal structures are not limited to these. Among these, a negative electrode active material having a P2 type structure is particularly preferable. A typical crystal system having a P2 type structure is attributed to the space group P6 ₃ / mmc because of its symmetry. In addition, when the symmetry further decreases, it may be assigned to an orthorhombic space group such as Cmcm and further to a monoclinic space group such as C2 / m, but the basic structure is classified as P2 type. It is a layered structure. However, P3 type and O3 type stacking faults may occur using the P2 type structure as a base structure.
Whether or not the metal oxide of the present invention has a P2 type structure can be confirmed by XRD measurement. Specifically, it can confirm by the method as described in an Example.

(Average primary particle size)
The average diameter (average primary particle diameter) of the metal oxide of the present invention is not particularly limited, but the lower limit is preferably 0.1 μm or more, more preferably 0.2 μm or more, and the upper limit is preferably 20 μm. Below, more preferably 10 μm or less. If the average primary particle diameter exceeds the above upper limit, the specific surface area may decrease, so that there is a possibility that battery characteristics such as rate characteristics and output characteristics may be decreased. If the lower limit is not reached, there is a possibility that problems such as inferior reversibility of charge / discharge due to the undeveloped crystals.
In addition, the average primary particle diameter in this invention is an average diameter observed with the scanning electron microscope (SEM), and is used as an average value of about 10-30 primary particle diameters using the SEM image of 30,000 times. Can be sought.

(Median diameter (secondary particles))
The median diameter (50% cumulative diameter (D ₅₀ )) of the secondary particles of the metal oxide of the present invention is not particularly limited, but is usually 2 μm or more, preferably 2.5 μm or more, most preferably 4 μm or more, and usually 50 μm. Hereinafter, it is preferably 30 μm or less, and most preferably 20 μm or less. If the median diameter is less than this lower limit, there is a possibility that a problem occurs in the coating property at the time of forming the negative electrode active material layer, and if it exceeds the upper limit, battery performance may be lowered.

(BET specific surface area)
The BET specific surface area of the metal oxide of the present invention is not particularly limited, but is usually 0.1 m ² / g or more, preferably 0.2 m ² / g or more, most preferably 0.3 m ² / g or more, usually 10 m. ² / g or less, preferably 5 m ² / g or less, more preferably 3 m ² / g or less, still more preferably 2 m ² / g or less, and most preferably 1 m ² / g or less. If the BET specific surface area is smaller than this range, the battery performance is likely to be lowered, and if it is larger, the bulk density is difficult to increase, and there is a possibility that a problem is likely to occur in the applicability when forming the negative electrode active material layer.

(Tap density)
The tap density of the metal oxide of the present invention is not particularly limited, but is usually 0.8 g / cc or more, preferably 1 g / cc or more, more preferably 1.4 g / cc or more, most preferably 1.5 g / cc or more. In general, it is 3.0 g / cc or less, preferably 2.8 g / cc or less, and most preferably 2.7 g / cc or less. While it is preferable for the tap density to exceed this upper limit for improving powder filling properties and electrode density, the specific surface area may be too low, and battery performance may be reduced. If the tap density is below this lower limit, there is a possibility that the powder filling property and the negative electrode preparation may be adversely affected.
The tap density in the present invention is determined as the powder packing density when 5 to 10 g of metal oxide powder is put in a 10 ml measuring cylinder and tapped 200 times with a stroke of 20 mm.

(Method for producing metal oxide)
The metal oxide of the present invention is not particularly limited as long as it has a layered crystal structure and a transition metal layer containing tetravalent Ti. For example, a raw material that becomes a Na source, a raw material that becomes a Li source or Mg source, and a raw material that becomes a Ti source are mixed so as to have the composition ratio described in the examples, and are manufactured by firing in an air atmosphere. In addition, by appropriately adjusting the composition ratio of Ti, Na, etc., a metal oxide having a layered crystal structure having a transition metal layer containing tetravalent Ti can be produced. In addition, it is possible to add a step of subjecting the obtained compound to atmospheric release or water washing treatment. Hereinafter, the manufacturing method will be described more specifically, but it is needless to say that the manufacturing method is not limited to these methods.

The type of raw material used for producing the metal oxide of the present invention is not particularly limited, but includes oxides, hydroxides, oxyhydroxides, hydrogencarbonates, carbonates, nitrates containing metal elements to be introduced. Examples thereof include organic acid salts such as sulfates, halides and oxalates, and alkoxides.
For example, examples of the Ti-containing compound serving as the Ti source include anatase TiO ₂ , rutile TiO ₂ , and Ti (OC ₂ H ₅ ) _4. Anatase TiO ₂ is preferable from the viewpoints of handling and reactivity. .
Examples of the Na-containing compound that serves as the Na source include Na ₂ CO ₃ , NaHCO ₃ , and Na ₂ O ₂ , and Na ₂ CO ₃ is preferable from the viewpoint of handleability.
Examples of the Mg-containing compound that serves as the Mg source include MgO, Mg (OH) ₂ , and MgCO ₃ , and MgO is preferable from the viewpoint of handleability.

In order to adjust the metal oxide of the present invention to a target composition / crystal structure, a Ti-containing compound and a compound containing a metal element to be introduced are weighed to a target ratio and mixed.
For _example, when manufacturing _{Na 0.68 Li 0.23 Ti 0.77 O 2} , Na 2 CO 3, Li 2 CO 3, TiO 2 molar ratio of 0.34: 0.115: it becomes 0.77 And mixing after weighing.
In addition, for mixing Ti-containing compounds and the like, industrially usual apparatuses such as a ball mill, a V-type mixer, a stirrer, and a dyno mill can be used. The mixing at this time may be either dry mixing or wet mixing. Moreover, you may obtain the mixture of the metal containing compound of a predetermined composition by the crystallization method. Furthermore, you may obtain the mixture with a sodium compound, after obtaining the composite metal carbonate or hydroxide of a predetermined composition by a coprecipitation method.
Specific mixing conditions are not particularly limited. For example, when a wet ball mill is used, the rotation speed is usually 10 to 1000 rpm, and the mixing time is 1 to 24 hours.

  The firing conditions for producing the metal oxide of the present invention should be appropriately set according to the composition of the metal oxide and the like. Usually, the firing temperature is 700 to 1000 ° C., and the firing time is 2 to 24 hours. is there. A firing temperature of 750 ° C. or higher is preferable for suppressing the generation of excessive stacking faults, and a baking temperature of 900 ° C. or lower is preferable for reducing the primary particle size. Further, if the firing time is 12 hours or more, it is preferable because a uniform chemical composition of single particles is obtained. If the firing time is 24 hours or less, crystal growth can be performed while maintaining stacking faults at a low temperature. This is preferable because it becomes possible.

  Examples of the atmosphere during firing include an inert atmosphere such as nitrogen and argon: an oxidizing atmosphere such as air, oxygen, oxygen-containing nitrogen, and oxygen-containing argon: and hydrogen-containing nitrogen containing 0.1 to 10% by volume of hydrogen. Any reducing atmosphere such as hydrogen-containing argon containing 0.1 to 10% by volume of hydrogen may be used. In order to fire in a strong reducing atmosphere, an appropriate amount of carbon may be contained in a mixture of metal-containing compounds and fired. In order to obtain Ti in a tetravalent state, the atmosphere during firing is preferably an oxidizing atmosphere such as air. When firing in a reducing atmosphere, it is preferable to subsequently perform firing in an air atmosphere or release to the atmosphere.

When a compound that can be decomposed and / or oxidized at a high temperature, such as a hydroxide, carbonate, nitrate, sulfate, halide, or oxalate, is used as a raw material metal-containing compound, before performing the above firing, The metal-containing compound may be calcined in the temperature range of 200 to 500 ° C. to obtain an oxide or crystal water may be removed. The atmosphere in which the calcination is performed may be an inert gas atmosphere, an oxidizing atmosphere, or a reducing atmosphere. Moreover, you may grind | pulverize and use the calcined material after calcining.

  In addition, it may be preferable to adjust the particle size by optionally subjecting the composite metal oxide obtained as described above to pulverization and classification using a ball mill, a jet mill or the like. Moreover, you may perform baking twice or more. Further, a surface treatment such as coating the particle surface of the composite metal oxide with an inorganic substance containing W, Mo, Zr, Si, Y, B or the like may be performed. The composite metal oxide preferably has a crystal structure that is not a tunnel structure.

<Anode active material for sodium ion battery / Anode for sodium ion battery>
The metal oxide of the present invention is a particularly suitable material as a negative electrode active material for a sodium ion battery. However, the negative electrode active material for a sodium ion battery containing the metal oxide of the present invention (hereinafter “the negative electrode active material of the present invention”). In addition, a negative electrode for a sodium ion battery (hereinafter sometimes abbreviated as “negative electrode of the present invention”) containing the negative electrode active material of the present invention is also an embodiment of the present invention.
The negative electrode of the present invention is not limited as long as it contains the metal oxide of the present invention. Usually, the negative electrode active material layer containing a negative electrode active material, a conductive agent, a binder and the like is provided on the surface of the current collector. Is formed.

  The negative electrode active material of the present invention may contain other known negative electrode active materials as long as it contains the above-described metal oxide. Examples thereof include carbon materials such as natural graphite, artificial graphite, coke, hard carbon, carbon black, pyrolytic carbon, carbon fiber, and organic polymer compound fired body that can occlude and desorb sodium ions. The shape of the carbon material may be any of a flake shape such as natural graphite, a spherical shape such as mesocarbon microbeads, a fibrous shape such as graphitized carbon fiber, or an aggregate of fine powder. Here, the carbon material may play a role as a conductive material.

  Although content of the negative electrode active material in a negative electrode active material layer is not specifically limited, It is preferable that it is 80 to 95 weight%.

  Examples of the binder include polyvinylidene fluoride (hereinafter sometimes referred to as PVDF), polytetrafluoroethylene (hereinafter sometimes referred to as PTFE), tetrafluoroethylene, hexafluoropropylene, and vinylidene fluoride copolymer. Examples thereof include a copolymer, a hexafluoropropylene / vinylidene fluoride copolymer, and a tetrafluoroethylene / perfluorovinyl ether copolymer. These may be used alone or in combination of two or more. Other examples of the binder include polysaccharides such as starch, methylcellulose, carboxymethylcellulose, hydroxymethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, carboxymethylhydroxyethylcellulose, and nitrocellulose and derivatives thereof. Examples of usable binders include inorganic fine particles such as colloidal silica. Although content of the binder in a negative electrode active material layer is not specifically limited, It is preferable that it is 5-10 mass%.

  Examples of the current collector include foil, mesh, expanded grid (expanded metal), punched metal, and the like using a conductive material such as nickel, aluminum, copper, and stainless steel (SUS). The mesh opening, wire diameter, number of meshes, etc. are not particularly limited, and conventionally known ones can be used. The general thickness of the current collector is 5 to 30 μm. However, a current collector having a thickness outside this range may be used.

The size of the current collector is determined according to the intended use of the battery. If a large electrode used for a large battery is manufactured, a current collector having a large area is used. If a small electrode is produced, a current collector with a small area is used.

  As a method for producing a negative electrode, first, a negative electrode active material, a conductive material, a binder, and an organic solvent are mixed to prepare a negative electrode active material slurry. Examples of organic solvents that can be used here include amines such as N, N-dimethylaminopropylamine and diethyltriamine; ethers such as ethylene oxide and tetrahydrofuran; ketones such as methyl ethyl ketone; esters such as methyl acetate; dimethylacetamide And aprotic polar solvents such as N-methyl-2-pyrrolidone.

  Next, the negative electrode active material slurry is coated on the negative electrode current collector, and dried and pressed to fix the slurry. Here, examples of the method of coating the negative electrode active material slurry on the negative electrode current collector include a slit die coating method, a screen coating method, a curtain coating method, a knife coating method, a gravure coating method, and an electrostatic spray method. be able to.

  As a method for forming the negative electrode active material layer on the negative electrode current collector, in addition to the above method, a mixture of the negative electrode active material, the conductive material, and the binder is placed on the negative electrode current collector, and pressure molding is performed. It is also possible to do it.

<Sodium ion battery>
The sodium ion battery provided with the negative electrode of the present invention is also an embodiment of the present invention (hereinafter may be abbreviated as “sodium ion battery of the present invention”). The sodium ion battery of the present invention is not particularly limited as long as it includes the above-described negative electrode, but generally includes a positive electrode capable of inserting and extracting sodium ions and an electrolyte in addition to the negative electrode.

[Positive electrode]
The positive electrode includes a current collector and a positive electrode active material layer formed on the surface of the current collector. The positive electrode active material layer includes a positive electrode active material, a conductive material, and a binder.

Although the kind of positive electrode active material will not be specifically limited if it can be used for a sodium ion battery, Specifically, a layered active material, a spinel type active material, an olivine type active material etc. can be mentioned. For _{example, NaFeO 2, NaNiO 2, NaCoO} 2, NaMnO 2, NaVO 2, Na (Ni X Mn 1-X) O 2 (0 <X <1), Na (Fe X Mn 1-X) O 2 (0 < X <1), NaVPO ₄ F, Na ₂ FePO ₄ F, Na ₃ V ₂ (PO ₄ ) _{3 and the} like.

  Although content of the positive electrode active material in a positive electrode active material layer is not specifically limited, It is preferable that it is 80-95 mass%.

  As the method for forming the positive electrode active material layer on the current collector and the kind of the conductive material, the binder and the like in the positive electrode, the same ones and methods used when manufacturing the negative electrode can be adopted.

[Electrolytes]
The electrolyte is not particularly limited, and either a general electrolytic solution or a solid electrolyte can be used. Examples of the electrolyte include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, isopropyl methyl carbonate, vinylene carbonate, 4-trifluoromethyl-1,3-dioxolan-2-one, 1,2-di ( Carbonates such as methoxycarbonyloxy) ethane; 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropyl methyl ether, 2,2,3,3-tetrafluoropropyl difluoromethyl ether, tetrahydrofuran, 2-methyl Ethers such as tetrahydrofuran; esters such as methyl formate, methyl acetate and γ-butyrolactone; nitriles such as acetonitrile and butyronitrile; N, N-dimethylformamide, N, N Amides such as dimethylacetamide; carbamates such as 3-methyl-2-oxazolidone; sulfur-containing compounds such as sulfolane, dimethyl sulfoxide and 1,3-propane sultone; or a fluorine substituent further introduced into the above organic solvent Things can be used. Usually, two or more of these are mixed and used as the organic solvent.

  Among the above electrolytic solutions, it is preferable to employ a non-aqueous solvent consisting essentially of a saturated cyclic carbonate (excluding the sole use of ethylene carbonate) or a mixed solvent of a saturated cyclic carbonate and a chain carbonate. In particular, when any one of these non-aqueous solvents is employed and hard carbon is employed as the negative electrode active material, the sodium ion secondary battery has excellent charge / discharge efficiency and charge / discharge characteristics. You may use the known additive currently used for the lithium ion battery as needed. In particular, fluoroethylene carbonate (FEC) is preferred as an additive.

  Here, “substantially” means a non-aqueous solvent composed only of a saturated cyclic carbonate (excluding the single use of ethylene carbonate), a non-aqueous solvent composed of a mixed solvent of a saturated cyclic carbonate and a chain carbonate. In the range which does not affect the performance of the sodium ion secondary battery such as the charge / discharge characteristics, it means that the solvent includes the solvent contained in the non-aqueous solvent used in the present invention.

  Among the saturated cyclic carbonates, use of propylene carbonate is preferable. Of the mixed solvents, it is preferable to use a mixed solvent of ethylene carbonate and diethyl carbonate or a mixed solvent of ethylene carbonate and propylene carbonate.

  As an electrolyte, an electrolyte salt that can be used when an electrolytic solution is employed is not particularly limited, and an electrolyte salt that is generally used for sodium secondary batteries can be used.

Examples of the electrolyte salt generally used in sodium secondary batteries include NaClO ₄ , NaPF ₆ , NaBF ₄ , CF ₃ SO ₃ Na, NaAsF ₆ , NaB (C ₆ H ₅ ) ₄ , CH ₃ SO ₃ Na, Examples thereof include NaN (SO ₂ CF ₃ ) ₂ , NaN (SO ₂ C ₂ F ₅ ) ₂ , NaC (SO ₂ CF ₃ ) ₃ , NaN (SO ₃ CF ₃ ) _{2 and the} like. In addition to the Na salt, an Li salt may be used as the electrolyte salt. In addition, 1 type may be used among the said electrolytes, and may be used in combination of 2 or more type.

The concentration of the electrolyte salt in the electrolytic solution is not particularly limited, but the concentration of the electrolyte salt is preferably 3 to 0.1 mol / l, and more preferably 1.5 to 0.5 mol / l. .

As the solid electrolyte, for example, an organic solid electrolyte such as a polyethylene oxide polymer compound, a polymer compound containing at least one of a polyorganosiloxane chain or a polyoxyalkylene chain can be used. Moreover, what is called a gel type which hold | maintained the nonaqueous electrolyte solution in the high molecular compound can also be used. _{Further, Na 2 S-SiS 2,} Na 2 S-GeS 2, NaTi 2 (PO 4) 3, NaFe 2 (PO 4) 3, Na 2 (SO 4) 3, Fe 2 (SO 4) 2 (PO 4 ), Fe ₂ (MoO ₄ ) ₃ or other inorganic solid electrolytes may be used.

[Structure of sodium secondary battery]
The structure of the sodium secondary battery of the present invention is not particularly limited, and when distinguished by form and structure, any conventionally known form such as a stacked (flat) battery, a wound (cylindrical) battery, etc.・ Applicable to structures. Further, when viewed in terms of electrical connection form (battery structure) in the sodium ion secondary battery, it can be applied to both (internal parallel connection type) batteries and bipolar (internal series connection type) batteries. .

  Hereinafter, the present invention will be described in detail with reference to examples. In addition, this invention is not limited to the Example shown below at all.

Example 1
Na ₂ CO ₃ , Li ₂ CO ₃ , TiO ₂ (average particle size: 25 nm, manufactured by Aldrich) as a raw material were weighed so as to have a molar ratio of 0.34: 0.115: 0.77, and then wet ball mill Was mixed for 12 hours at 600 rpm. After pellet-molding the obtained mixture, it was calcined at 850 ° C. for 0.5 hours in an air atmosphere and baked at 900 ° C. for 24 hours, and then quenched to obtain Na _0.68 Li _0.23 Ti _{0. 77} O ₂ (Compound 1) was synthesized.
As a result of XRD analysis of Compound 1, it was confirmed that it had a P2 type layered structure belonging to the space group P6 ₃ / mmc. The XRD measurement result of Compound 1 is shown in FIG.

Using compound 1 of this example, compound 1: PVdF (polyvinylidene fluoride): AB (acetylene black) = 8: 1: 1 (mass ratio) was mixed, and slurried using NMP, current collection The electrode was produced by applying to body Al. A battery using 1M NaClO ₄ / PC (propylene carbonate) + FEC (fluoroethylene carbonate) (volume ratio: 98: 2) as an electrolyte and using Na metal as a counter electrode was produced.
A charge / discharge test was performed in a voltage range of 0V-2V at a current density of 10 mA / g. The charge / discharge curve up to 40 cycles is shown in FIG. A charge / discharge capacity of 84 mAh / g was obtained. The initial irreversible capacity was 41 mAh / g. This battery showed almost no capacity deterioration even after repeated 40 cycles or more.

(Example 2)
After impregnating compound 1 obtained in Example 1 in distilled water, compound 2 was obtained by drying at 80 ° C. for 1 day. As a result of XRD analysis of Compound 2, it was confirmed that the P2-type layered structure belonging to the space group P6 ₃ / mmc was maintained, and it was found that the compound was a stable compound that could be handled even in an aqueous solution. The XRD measurement result of Compound 2 is shown in FIG.

Example 3
Na ₂ CO ₃ , MgO, and anatase TiO ₂ (average particle size 325 mesh, manufactured by Aldrich) were weighed so as to have a molar ratio of 2: 1: 2, and then mixed using a mortar. After pellet-molding the mixture, Na _2/3 Mg _1/3 Ti _2/3 O ₂ (Compound 3) was synthesized by calcining at 850 ° C. for 30 minutes in an air atmosphere and firing at 1000 ° C. for 24 hours. As a result of XRD analysis of Compound 3, it was confirmed that it mainly contains an O3 type layered compound belonging to the space group R-3m. The XRD measurement result of Compound 3 is shown in FIG.
A battery was fabricated in the same manner as in Example 1 except that Compound 3 was used. As a result of the charge / discharge test, a capacity of 64 mAh / g was obtained. A charge / discharge curve is shown in FIG. The initial irreversible capacity was 45 mAh / g. This battery showed a capacity of 62 mAh / g even after 20 cycles, and was confirmed to be excellent in cycle stability.

Example 4
Na ₂ CO ₃ , MgO, and anatase TiO ₂ (average particle size: 25 nm, manufactured by Aldrich) were weighed so as to have a molar ratio of 2: 1: 2, and then mixed using a mortar. After the mixture was formed into a pellet, Na _2/3 Mg _1/3 Ti _2/3 O ₂ (Compound 4) was synthesized by calcining at 800 ° C. for 2 hours and firing at 1000 ° C. for 6 hours in an air atmosphere. Compound 4
As a result of XRD analysis, it was confirmed to have an O3 type layered compound belonging to the space group R-3m. The XRD measurement result of Compound 4 is shown in FIG.
A battery was fabricated in the same manner as in Example 1 except that Compound 4 was used. As a result of the charge / discharge test, a capacity of 74 mAh / g was obtained. The initial irreversible capacity was 55 mAh / g. This battery showed a capacity of 70 mAh / g even after 4 cycles, and was confirmed to be excellent in cycle stability. A charge / discharge curve is shown in FIG.

(Example 5)
A battery was fabricated in the same manner as in Example 1 except that Compound 1 used in Example 1 was used, 1M LiPF ₆ / EC (ethylene carbonate) + DMC (dimethyl carbonate) was used as the electrolyte, and Li was used as the counter electrode. As a result of the charge / discharge test, a capacity of 166 mAh / g was obtained. The initial irreversible capacity was 73 mAh / g. This battery showed 142 mAh / g after 5 cycles, and a decrease in capacity was observed. From this result, it was confirmed that when the Li ion was contained as the electrolyte, it acted as a negative electrode.

(Comparative Example 1)
A negative electrode was prepared in the same manner as in Example 1 except that anatase TiO ₂ (average particle size: 25 nm, manufactured by Aldrich) was used and polyacrylic acid was used as the binder. A battery was prepared using 1M NaPF ₆ / PC (propylene carbonate) as the electrolyte and using Na as the counter electrode. As a result of the charge / discharge test, an initial charge capacity of 318 mAh / g was obtained, and the reversible capacity was 156 mAh / g. The initial irreversible capacity was as large as 162 mAh / g. This battery showed 157 mAh / g after 20 cycles and no reduction in capacity was observed.

The use of the sodium ion battery provided with the negative electrode produced using the metal oxide of the present invention is not particularly limited, and can be used for various known uses. Specific examples include notebook computers, pen input computers, mobile computers, electronic book players, mobile phones, mobile faxes, mobile copy, mobile printers, headphone stereos, video movies, LCD TVs, handy cleaners, portable CDs, minidiscs, and transceivers. , Electronic notebook, calculator, memory card, portable tape recorder, radio, backup power supply, stationary power supply, motor, lighting equipment, toys, game equipment, clock, strobe, camera, pacemaker, power tool, power source for bicycles and motorcycles, Examples thereof include an automobile power source, an orbital vehicle power source, and a satellite power source.

Claims (6)

  A metal oxide having a layered crystal structure, wherein the crystal structure has a transition metal layer containing tetravalent Ti. The metal oxide according to claim 1, wherein the composition formula is represented by the following formula (I).
_{Na x A a Ti b M c} O 2 ··· (I)
(In Formula (I), x <1, 0 <a ≦ 1/3, 2/3 ≦ b <1, 0 ≦ c <1/3, a + b + c = 1. A is selected from Li and Mg. And at least one element selected from V, Cr, Fe, Mn, Co, Ni, Cu, Ca, Sn, and Zn, and constitutes a transition metal layer Ti can be substituted.)   The metal oxide according to claim 1 or 2, wherein the crystal structure is a P2-type structure, an O3-type structure, or a P3-type structure.   A negative electrode active material for a sodium ion battery, comprising the metal oxide according to any one of claims 1 to 3.   A negative electrode for a sodium ion battery, comprising the negative electrode active material for a sodium ion battery according to claim 4.   A sodium ion battery comprising the sodium ion battery negative electrode according to claim 5.

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